Chapter Two
The biological bases of human communicative behavior
Outline
Introduction
Language & the brain: A historical perspective
- Early neurolinguistic observations
- Localizations of function (nineteenth & twentieth century neurology)
Functional neuroanatomy & neuropathology
- Neuroanatomical Structures Involved in Speech and Language
- How Speech is Controlled by the Brain
- Neural Cells and Their Connections: the Ultimate Basis of All Behavior
- What Can Go Wrong With the Brain: Neuropathology
- Examining the Consequences of Cortical Damage
Lateralization of function
- Putting Half the Brain to Sleep: the Wada Test
- Splitting Apart the Hemispheres: Commissurotomy
- Taking out Half the Brain: Hemispherectomy
- Listening with Both Ears: the Dichotic Listening Technique
- What Functions Reside in the Nondominant Hemisphere?
- When Sign Language Users Become Aphasic
Intrahemispheric localization of function
- Measuring Electrical Activity in the Brain
- Measuring Blood Flow in The Brain
- The Role of Subcortical Structures in Speech and Language
Ways of viewing the relationship between brain & language
Key Concepts From the Book
I. The field of neurolinguistics is primarily concerned with the role that anatomical and physiological structures play in the production and comprehension of speech and language. Neurolinguists are particularly interested in exploring where language functions reside within the brain, and how the brain processes and produces language. (Page 52)
II. The relationship between language and brain function has been studied since at least 3000 BC. Insights into this relationship have historically grown out of studies of people with head injuries and strokes. (Page 52-53)
III. By the eighteenth century, almost all known language and speech disorders had already been described. Sixteenth century medical scholar Johann Schenk Von Grafenberg (1530-1598) was the first to point out that language disturbances due to brain injury were not the result of paralysis of the tongue (dysarthria). A contemporary scholar, G. Mercuriale described a patient with alexia -- he could write, but not read. Other scholars described patients with jargon aphasia, jargon agraphia and bilingual aphasia. (Page 53)
IV. The first scientist to theorize that language abilities might be located in a particular part of the brain was neuroanatomist Franz Josef Gall (1758-1828). Gall was also the first to point out the difference between white and gray matter. His belief in the localization of various functions within the brain led him to found the school of cranioscopy, otherwise known as phrenology. (Page 54)
V. During the nineteenth century, Pierre Paul Broca (1824-1880) discovered that articulate language is typically located in the left third frontal convolution in the left hemisphere. This area is known as Broca's area. Patients with damage to Broca's area typically show symptoms of Broca's aphasia. Broca's aphasics have labored, ungrammatical speech and exhibit severe word retrieval problems. (Pages 54-55)
VI. Broca also discovered that lateralization of language is loosely connected to handedness, relating both to the precocious development of the left hemisphere. He also commented upon the plasticity of the young brain in response to trauma, asserting that young children with damage to Broca's area may learn to talk. (Page 56)
VII. More recent research into the response of young brains to trauma has shown that they are indeed, flexible. Lenneberg (1967) posited that there is a critical period for language acquisition during which language learning occurs rapidly and brain damage produces no lasting communicative disorder. Further support for the idea of a critical period for language learning comes from studies of extremely neglected children such as Genie, a child who was provided no verbal interaction between the ages of 2 and 13 * , and was subsequently unable to learn fluent, expressive speech. (Pages 56-57)
VIII. Carl Wernicke (1848-1904) discovered another area of the brain important in language abilities. Wernicke was interested in an area contiguous to the highest cortical area associated with hearing. Damage to this (Wernicke's) area brings about Wernicke's aphasia. Wernicke's aphasics speak fluently and their sentences appear to have discernible grammatical structure. What they say, however, often makes little sense, and is often filled with nonsense words (neologisms). Wernicke's aphasics also have severe comprehension problems, and (unlike Broca's aphasics) often are not aware that they are ill. (Pages 57-59)
IX. Wernicke, working with neurologist Ludwig Lichtheim, produced a classification of observed aphasias as well as those logically possible. The Wernicke-Lichtheim model is based on neuroanatomical considerations and predicts the communicative consequences of injury to various parts of the brain. Although the model has had detractors, it has proven remarkably resilient, despite being somewhat simplistic. Considered the "classical model" of aphasias, it constitutes the first approximation of the final goal of localizing language functions within the brain. (Page 59)
X. The brain, the uppermost portion of the Central Nervous System (CNS), is housed within the cranium, protected by three layers of membranes (meninges) and floating in cerebral spinal fluid. The most rostral structure is the cerebral cortex. This is divided into two hemispheres connected by a number of fiber tracts (commissures), the largest being the corpus callosum. Although the hemispheres appear identical, they are not. The brain itself is composed of alternating layers of white matter (nerve fibers) and grey matter (nerve cells). Although it averages only about 3.5 pounds, it utilizes 1/5 of the body's blood supply. (Pages 60-61)
XI. At the center of the brain is the diencephalon, a mass of neurons. This structure serves as a way station for most incoming sensations, and provides motor feedback to the cortex. Damage to the dorsal thalamus (one of its components) can produce dysarthria as well as aphasia. Damage to the basal ganglia, the next layer of grey matter, can result in hypokinesia (such as in Parkinson's disease) or hyperkinesea (as in Huntington's chorea.) Damage to the cerebellum, the next layer of grey matter, can produce dysarthria and ataxia.(Pages 62-63)
XII. The brain stem lies at the base of the brain. It consists of the midbrain, pons, and medulla. The brain stem controls the functioning of the heart and lungs. The remainder of the CNS consists of the spinal cord housed within the vertebral column. The human spinal cord is nonautonomous. (Page 63)
XIII. Various elements of the peripheral nervous system are also important for language function. The cranial nerves are important in controlling vision, smell, hearing, and facial sensation. These nerves play an important role in phonation. (Pages 63-64)
XIV. Speech involves around 100 muscles. At a normal speech rate of about 14 sounds per second, this would mean 140,000 neuromuscular events per second. Movement in primates (including humans) is controlled by at least three distinct motor systems: one controls individual movements of the digits; the second, independent movements of hands and arms; the third, posture and bilateral trunk and limb movements. The motor control system of speech is probably the first system. (Pages 64-65)
XV. The fine motor control system corresponds to the fibers of the pyramidal tract. Prior to medically necessary brain surgery, neurosurgeons must locate important areas of the brain via electrical stimulation. By doing this, neurologists have been able to locate areas responsible for the function of various body parts. A proportionally large area of the brain is devoted to the control of the head and face. Fibers from this area travel downwards, making contact with the cranial nerves, which are involved in various aspects of speech production. (Pages 64-65)
XVI. The brain is composed of neurons (nerve cells) and glia (glue cells). Electrical impulses are transmitted from one neuron to another across a gap (synapse) through chemical agents called neurotransmitters. Many things can go wrong in such a complex system. Neuropathology examines the consequences of brain damage and disease. These include:
A. Cerebrovascular disease
B. Trauma, tumors and hydrocephalus
C. Multiple sclerosis
D. Parkinsonism & Huntington's chorea
E. Myasthenia gravis (Pages 67-68)
XVII. Damage to various language areas of the brain can produce many different types of aphasia in which different skills are lost or retained. Damage to the third frontal convolution causes Broca's (expressive or nonfluent) aphasia.(). Lesions in the motor strip can cause dysarthria. Wernicke's or cortical sensory aphasia (also receptive or fluent aphasia) is produced by damage to the posterior third of the first temporal gyrus. Damage to the angular gyrus can produce anomia. A disruption between Heschl's gyrus (responsible for hearing) and Wernicke's area might produce subcortical aphasia or pure word deafness: the inability to understand spoken language, while retaining the ability to speak, read and write. (Pages 68-70)
XVIII. Other types of aphasia include subcortical motor aphasia, conduction aphasia, paroxysmal aphasia, global aphasia, and mixed transcortical aphasia, in which the patient cannot produce spontaneous speech but can repeat what is said to her. Damage to some areas of the brain also may produce dementia or agnosia, in which language abilities are spared, but thought processes are disrupted. In some cases of global aphasia, although all language skills are absent, ideation survives, arguing for the separability of linguistic and cognitive competence. (Pages 70-73)
XIX. The left and right hemispheres of the brain differ in function. Although in the normal brain both hemispheres are involved in language function, bihemispheric involvement may not be necessary for reasonably good functioning. (Page 73)
XX. The Wada test involves the injection of sodium amytol into the internal or common carotid arteries, producing contralateral hemiplegia and deactivating the ipsilateral hemisphere. Experiments using this test have shown that most right handed individuals are left-lateralized for language. Many left handers and ambidextrals have bilateral representation of language, as do many people who suffered early left hemisphere damage. A very small number of right handed people have right dominance for language. (Page 73-74)
XXI. Damage to the perisylvian language and speech region in children under five years of age may result in a shift in the lateralization of language. Damage after five years rarely produces a shift in lateralization. (Page 75)
XXII. Commissurotomy is an operation that destroys the corpus callosum and thus disconnects the two hemispheres. Studies of "split-brain" subjectshave revealed that although usually only the dominant hemisphere can produce verbal output, many language abilities reside in the nondominanthemisphere.(Pages 75-77)
XXIII. Hemispherectomy is the removal of half the brain: a radical surgery sometimes performed on patients with severe neuropathologies. In cases of dominant hemispherectomy in adults, verbal output is very severely affected. If the surgery is performed on a very young child (under the age of about five), gradual recovery of language abilities appears to be almost complete.(Pages 77-78)
XXIV. More recent studies have shown that removal of half the brain does take its toll. Studies of children who are hemidecorticates show they have trouble with various language tasks such as detecting syntactic anomalies. (Pages 78-79)
XXV. One of the problems with studying brain damaged individuals in order to determine the location of language abilities within the brain is that damage in one area may have consequences for functioning in another. Neuroimaging techniques have found metabolic abnormalities in otherwise healthy tissue located at a distance from the lesion site. (Page 79)
XXVI. Dichotic listening is a technique developed to study the brains of healthy individuals. Different stimuli are presented to the left and right ears of thesubject, who is then asked to report on what he heard. In tasks such as this, the left hemisphere processes words, numbers and nonsense syllables more quickly and accurately than the right hemisphere. The right hemisphere is more accurate when dealing with music, human non-speech stimuli, and visual-spatial processing tasks. (Pages 79-80)
XXVII. Other functions that may reside in the right hemisphere include the ability to understand metaphorical and figurative language, as well as the ability to remember sequences of events or draw a moral from a story. Other paralinguistic functions, such as the ability to read facial expressions and understand stress and intonation, also appear to inhabit the right hemisphere. (Pages 80-82)
XXVIII. Some evidence, such as the fact that women tend to recover from aphasia more completely than men, suggests that women have their language abilities more diffusely organized within the brain than do men. These findings are further supported by recent research using functioning magnetic resonance imaging. (Page 82 & 91)
XXIX. When sign language users suffer damage to language areas of the brain, they become aphasic in many of the same ways as users of spoken language.(Pages 82-83)
XXX. Techniques that attempt to locate functions precisely within the brain include examining brain activity with an electroencephalogram (EEG). Event related potentials (ERPs) have shown that the brain responds differently to tasks involving syntactic and semantic processing. (Page 83)
XXXI. Measurements of regional cerebral blood flow (rCBF), a technique pioneered by Broca in the late 1870s, shows which areas of the brain are active when the subject is asked to perform such tasks as listening, speaking and humming a song. A highly sophisticated method of measuring rFCB is a scanning technique called positron emission tomography (PET) that provides a three-dimensional representation of blood flow within the brain, and allows monitoring of subcortical structures. Analysis of PET scans on subjects performing various language tasks have shown that different areas of the brain are used for different grammatical functions. (Page 86-89)
XXXII. Subcortical structures also operate in speech and language functions. Damage to these structures can result in dysarthrias and semantic paraphasias. (Page 90)
XXXIII. Cognitive neuropsychology is a blend of neuropsychology and cognitive psychology. Cognitive neuropsychologists attempt to draw conclusions about normal cognitive functioning by studying brain injured individuals. The study of language within this approach is called linguistic aphasiology. The basic tenet of this discipline is that the mind is composed of a dissociable set of processing modules, which are held to be related to distinct areas of the brain. (Pages 92-93)
XXXIV. Linguistic aphasiologists are critical of the terminology of traditional aphasic syndromes, citing their lack of specificity in analysis of language dysfunctions, and the fact that patients do not neatly fall into any specific category. (Pages 93-94)
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